Contents

Properties

Supergiants have masses from 8 to 12 times the Sun (M☉) upwards, and luminosities from about 10,000 to over a million times the Sun (L☉). They vary greatly in radius, usually from 30 to 500, or even in excess of 1,000 solar radii (R☉). They are massive enough to begin core helium burning gently before the core becomes degenerate, without a flash, and without the strong dredge-ups that lower-mass stars experience. They go on to successively ignite heavier elements, usually all the way to iron. Also because of their high masses they are destined to explode as supernovae.

The Stefan-Boltzmann law dictates that the relatively cool surfaces of red supergiants radiate much less energy per unit area than those of blue supergiants; thus, for a given luminosity red supergiants are larger than their blue counterparts. Radiation pressure limits the largest cool supergiants to around 1,500 R☉ and the most massive hot supergiants to around a million L☉ (MV around -9). Stars near and occasionally beyond these limits become unstable, pulsate, and experience rapid mass loss.

Supergiants are categorized on the basis of their spectra. Supergiants occur in every spectral class from young blue class O supergiants to highly evolved red class M supergiants. Because they are enlarged compared to main-sequence and giant stars of the same spectral type, they have lower surface gravities and changes can be observed in their line profiles. Supergiants are also evolved stars with higher levels of heavy elements than main-sequence stars. This is the basis of the MK luminosity system which assigns stars to luminosity classes purely from observing their spectra. In addition to the line changes due to low surface gravity and fusion products, the most luminous stars have high mass-loss rates and resulting clouds of expelled circumstellar materials which can produce emission lines, P Cygni profiles, or forbidden lines. The MK system assigns stars to luminosity classes: Ib for supergiants; Ia for luminous supergiants; and 0 (zero) or Ia+ for hypergiants. In reality there is very much of a continuum rather than well defined bands for these classifications, and classifications such as Iab are used for intermediate luminosity supergiants. Supergiant spectra are frequently annotated to indicate spectral peculiarities, for example B2Iae or F8Iabpec.

Categorisation of stars

Although the term supergiant does not have a single concrete definition, there are several other categories of evolved star which are not, not always, or no longer, generally treated as supergiants. Definitions based solely on spectral type, solely on luminosity, or on criteria such as size, mass, composition, internal structure, or life stage, will all include somewhat different stars under the heading of supergiant.

Asymptotic-giant-branch (AGB) stars are highly evolved lower-mass red giants with luminosities almost as high as red supergiants, but because of their low mass, being in a different stage of development (helium shell burning), and their lives ending in a different way (planetary nebula and white dwarf rather than supernova), astrophysicists prefer to keep them separate. The dividing line becomes blurred at around 7–10 M☉ (or as high as 12 M☉ in some models1) where stars start to undergo limited fusion of elements heavier than helium. Specialists studying these stars often refer to them as super AGB stars, since they have many properties in common with AGB such as thermal pulsing. Others describe them as low-mass supergiants since they start to burn elements heavier than helium and can explode as supernovae.2 These intermediate stars develop oxygen–magnesium–neon cores that either lead to the rare oxygen–neon white dwarf or an electron-capture supernova.

Wolf–Rayet stars are also high-mass luminous evolved stars, hotter than most supergiants and smaller, visually less bright but often more luminous because of their high temperatures. They have spectra dominated by helium and other heavier elements, usually showing little or no hydrogen, which is a clue to their nature as stars even more evolved than supergiants. Just as the AGB stars occur in almost the same region of the HR diagram as red supergiants, Wolf–Rayet stars can occur in the same region of the HR diagram as the hottest blue supergiants and main-sequence stars.

The most massive and luminous main-sequence stars are almost indistinguishable from the supergiants they quickly evolve into. They have almost identical temperatures and very similar luminosities, and only the most detailed analyses can distinguish the spectral features that show they have evolved away from the narrow early O-type main-sequence to the nearby area of early O-type supergiants. Such early O-type supergiants share many features with WNLh Wolf–Rayet stars and are sometimes designated as slash stars, intermediates between the two types.

Luminous blue variables (LBVs) are a type of star that occur in the same region of the HR diagram as blue supergiants, but are generally classified separately. They are evolved, expanded, massive, and luminous stars, often hypergiants, but they have very specific spectral variability which defies the assignment of a standard spectral type. LBVs only observed at a particular time, or over a period of time when they are stable, may simply be designated as hot supergiants, or as candidate LBVs due to their luminosity.

Hypergiants are frequently treated as a different category of star from supergiants, although in all important respects they are just a more luminous category of supergiant. They are evolved, expanded, massive and luminous stars like supergiants, but at the most massive and luminous extreme, and with particular additional properties of undergoing high mass-loss due to their extreme luminosities and instability. Generally only the more evolved supergiants show hypergiant properties since their instability increases after high mass-loss and some increase in luminosity.

Some B(e) stars have temperatures and luminosities equivalent to blue supergiants, although other B(e) stars are clearly different. Some researchers distinguish the B(e) objects as separate from supergiants, while others prefer to define the particularly massive and luminous B(e) stars a subgroup of supergiants. The latter has become more common with the understanding that the B(e) phenomenon arises separately in a number of distinct types of stars, including some that are clearly just a phase in the life of supergiants.

Further types of variable stars, such as RV Tauri variables and PV Telescopii variables, are often described as supergiants. Although the rare RV Tau stars are frequently assigned spectral types with a supergiant luminosity class on account of their low surface gravity, they are lower-mass lower-luminosity post-AGB stars. Likewise the even rarer PV Tel variables are often described as supergiants, but have lower luminosities than supergiants and peculiar B(e) class spectra extremely deficient in hydrogen. Possibly they are also post-AGB objects, or perhaps "born-again" AGB stars.

The LBVs already mentioned are variable with multiple semi-regular periods and less predictable eruptions and giant outbursts. Although they are essentially supergiants in nature, extremely luminous, massive, evolved stars with expanded outer layers, they are so distinctive and unusual that they are often treated as a separate category without being referred to as supergiants or given a supergiant spectral type. Often their spectral type will be given just as "LBV" because they have peculiar and highly variable spectral features, with temperatures varying from about 8,000 K in outburst up to 20,000 K or more when "quiescent".

Evolution

O type main sequence stars and the most massive of the B type blue-white stars become supergiants. Because of their extreme masses they have short lifespans of 30 million years down to a few hundred thousand years.3 They are mainly observed in young galactic structures such as open clusters, the arms of spiral galaxies, and in irregular galaxies. They are less abundant in spiral galaxy bulges, and are rarely observed in elliptical galaxies, or globular clusters, which are composed mainly of old stars.

Supergiants develop when massive main sequence stars run out of hydrogen in their cores. They then start to expand, just like lower mass stars, but unlike lower mass stars, they begin to fuse helium in the core almost immediately. This means that they do not increase their luminosity as dramatically as lower mass stars and they progress nearly horizontally across the HR diagram to become red supergiants. Also, unlike lower mass stars, red supergiants are massive enough to fuse elements heavier than helium, so they do not puff off their atmospheres as planetary nebulae when their helium becomes depleted. Furthermore, they cannot lose enough mass to form a white dwarf, so will leave behind a neutron stars or black hole remnant, usually after a core collapse supernova explosion.

Stars more massive than about 40 M☉ cannot expand into a red supergiant. They burn too quickly and lose their outer layers too quickly, so they reach the blue supergiant stage, or perhaps yellow hypergiant, and then return to become hotter stars. The most massive stars, above about 100 M☉, hardly move at all from their position as O main sequence stars. These stars convect so efficiently that they mix hydrogen from the surface right down to the core. They continue to fuse hydrogen until it is almost entirely depleted throughout the star, then very rapidly evolve through a series of stages of very similar hot and luminous stars, If supergiants, slash stars, WNh stars, WN stars, and possibly WC or WO stars. They are expected to explode as supernovae but it is not clear how far they evolve before this happens. The existence of these supergiants still burning hydrogen in their cores may necessitate a slightly more complex definition of supergiant: a massive star with increased size and luminosity due to fusion products building up, but still with some hydrogen remaining.4

The first stars in the universe are thought to have been considerably brighter and more massive than the stars in the modern universe. These stars were part of the theorized population III of stars. Their existence is necessary to explain observations of elements other than hydrogen and helium in quasars. Although they may have been larger and more luminous than any supergiant known today, their structure was quite different, with reduce convection and less mass loss. Their very short lives are likely to have ended in violent photodisintegration or pair instability supernovae.

Supernova progenitors

Most type II supernova progenitors are thought to be red supergiants, while the less common type Ib/c supernovae are produced by hotter Wolf-Rayet stars that have completely lost more of their hydrogen atmosphere.5 Almost by definition, supergiants are destined to end their lives violently. Stars that are large enough to start fusing elements heavier than helium just do not seem to have any way to lose enough mass to avoid catastrophic core collapse, although some of them may collapse almost without trace into their own central black holes.

However, the simple "onion" models showing red supergiants inevitably developing to an iron core and then exploding have been shown to be much too simplistic. The progenitor for the unusual type II Supernova 1987A was a blue supergiant,6 thought to have already passed through the red supergiant phase of its life, and this is now known to be far from an exceptional situation. Much research is now focused on how blue supergiants can explode as a supernova and when red supergiants can survive to become hotter supergiants again.7

Well known examples

Supergiants are rare and short-lived stars, but their high luminosity means that there are many naked eye examples, including some of the brightest stars in the sky. Rigel is the brightest star in the constellationOrion and a typical blue-white supergiant, Deneb is the brightest star in Cygnus and a white supergiant, Delta Cephei is the famous prototype Cepheid variable and a yellow supergiant, while Betelgeuse and Antares are red supergiants. μ Cephei is one of the reddest stars visible to the naked eye and one of the largest in the galaxy. Rho Cassiopeiae is a naked eye variable, a yellow hypergiant, and one of the most luminous naked eye stars.